Control of emissions from motor vehicles is now an accepted and necessary automotive design consideration throughout the world. In the United States since about 1965 (and earlier in California) all motor vehicles sold have incorporated some form of emission control. Recently, more vehicles sold for use in countries other than the United States have also been designed to reduce pollutants. The speed with which engine design changes have taken place to satisfy pollution reduction requirements has been extraordinary. Not only have the engine and vehicle manufacturers had to engage in major expenditures for facilities, equipment and accelerated technical achievement, but similarly, the automotive service industry has been experiencing a major upheaval in the effort to provide continuing qualified engine malfunction diagnosis and maintenance capability.
Exhaust emission standards in the United States have become increasingly stringent. Concurrent with the emphasis on reducing emissions, the automotive service industry throughout the world was beginning to acquire exhaust analyzers for measurement of hydrocarbons (HC) and/or carbon monoxide (CO). Now, the majority of qualified automobile service centers, particularly in the United States, use HC/CO analyzers routinely as an important diagnostic aid and also to inspect or verify vehicle manufacturer specifications at idle. In a number of areas withing the United States, legislation requires service garages to have an approved HC/CO analyzer. CO analyzers have also become a mandatory part of emission controls programs in a number of countries other than the United States.
Service facilities have found emission analyzers a useful diagnostic/service device during routine service inspection. For example, the use of a CO analyzer for properly adjusting carburetor balance and air/fuel ratios is now standard practice. Hydrocarbon measurements as an indication of ignition problems, a malfunctioning exhaust or intake valve, etc., is also widely used as a quick method of screening vehicles for further diagnosis by conventional oscilloscope testers.
The stringent standards in the United States for 1975 have forced most automobile manufacturers to use catalytic converters on current production vehicles to provide adequate control of exhaust emissions of hydrocarbons and carbon monoxide. Unfortunately, the effective use of HC/CO exhaust gas analyzers as a diagnostic aid for vehicles equipped with catalytic converters is more complicated than for vehicles without converters. In fact, if the converter is working properly, engine diagnosis with HC/CO analyzers is extremely difficult unless the vehicle has an exhaust sampling port ahead of the converter. When the catalytic converter is functioning properly, it oxidizes essentially all of the HC and CO to CO.sub.2 and water vapor. Consequently, the concentrations in the exhaust are so low they cannot be measured with accuracy with existing "garage-type" instrumentation. The changes in raw exhaust concentrations of HC, for example, as a result of intermittent misfire or "lean-roll", no longer appear in the converted exhaust gases and the conventional exhaust gas analyzer loses value as a diagnostic tool.
The detection of a lean-roll condition is becoming of great concern not only to the service garage owner doing after sale service, but, to the automobile manufacturer and his dealers as well. The leaner the mixture at which the carburetor is set, the greater the economy when the engine is operating in the manner for which it was designed. A slight deviation, however, can put the engine in a lean-roll condition which increases both gasoline consumption and the emission of pollutants at the exhaust. The phenomenon of lean-roll can best be understood with reference to FIG. 1. In a typical engine 10 having cylinders 12, a common intake manifold 14 is connected from a carburetor 16 to the intake ports 18 of cylinders 12. Obviously, the distance from the two outside cylinders 12 is greater than the distance to the two inside cylinders 12. The variation in the distances that the gasoline/air mixture must travel through the manifold 14 to reach the various cylinders 12 causes a difference in the mixture at the various cylinders 12 even though produced by a common carburetor 16. The mixture at the inner cylinders 12 tends to be richer than the mixture at the outer cylinders 12. In order to ignite and burn properly in the cylinders 12 the gasoline to air ratio of the fuel mixture must be within certain high and low limits. As the mixture is made more lean (less gasoline to a fixed volume of air) a point will be reached where the mixture will not ignite.
Since, as stated earlier, the mixture at the two outer cylinders 12 tends to be leaner, as the mixture at the carburetor 16 is adjusted leaner, the two outer cylinders 12 will reach a point where they cyclically begin to misfire. This is called the "lean-roll" condition. As the mixture received drops below the critical level, the outer cylinders 12 misfire. This condition can occur individually or simultaneously as the exact mixture in any cylinder 12 at any instant is a function of many factors including the mixture at the carburetor at that instant (which may vary), the temperature of the manifold 14, whether the cylinder 12 fired on the last power cycle, and the amount of dilutants retained from the exhaust cycle during the intake stroke. Lean mixture can, of course, occur at all the cylinders 12 but we are concerned here with the slow roll phenomenon which is more correlatable to the outside cylinders (those furthest from the carburetor) in any engine.
In an application for United States Letters Patent titled IMPROVED ENGINE ANALYSIS APPARATUS filed concurrently herewith in the names of John D. Blanke et al. and assigned to the common assignee of this application, a misfire monitor is described which is capable of detecting lean-roll in an internal combustion engine by comparing the rate of change of O.sub.2 in the exhaust gases with respect to time (dO/dt) against a limit. Various factors associated with different engines, such as air blowing, make the value of dO/dt during an actual lean-roll condition a variable item. In order to prevent false indications, it is preferable to have the dO/dt-limit value comparison made particularly adapted to the engine under analysis. In production line situations of engine analysis, roadside checks by enforcement agencies involving numerous automobile types, or similar situations, it is desirable that any instrumentation minimize the requirements for operator intervention or decision making. Further, air leaks into the exhaust system can make the actual O.sub.2 content of an engine's exhaust be outside the range expected. In such instances, self-regulating equipment can be used to good advantage.
Therefore, it is the object of the present invention to provide a misfire monitor capable of automatically adjusting the parameters used in the testing for a lean-roll condition to a standard range whereby the misfire monitor can be used on various engine types with little or no operator intervention.